Portable information terminals such as mobile phones typically employ liquid crystal display devices as their display means. Also, since these mobile phones and others are battery-driven, reduced power consumption is an essential requirement. For this reason, permanent display contents (such as current time and battery status) are typically displayed in a reflective sub-panel. Also, as a recent trend, there is a demand for a main panel which is capable of handling both normal display and reflective permanent display.
Power consumption when driving a liquid crystal panel is primarily the amount of power consumed by a source driver serving as a data signal line driving circuit to drive source lines (data signal lines). The amount is given by the following expression:P∝f·C·V·V·n·m  (1)where P represents power consumption for driving the liquid crystal panel, f represents refreshing frequency, i.e., the number of refreshing (rewriting) procedures performed to a frame of pixel data per unit time. C represents a load capacitance driven by the source driver; V represents a drive voltage by the source driver; n represents a quantity of scanning lines; and m represents a quantity of source lines.
Contents of the permanent display are still images. In other words, the contents need not be updated. Based on this understanding, there is an attempt for further reduction of power consumption in the liquid crystal display device by decreasing the refreshing frequency for permanent display. However, decreasing the refreshing frequency results in creeping of pixel electrode potential due to leak current, etc. via switching elements such as thin-film transistors in the liquid crystal display device. Thus, as the refreshing frequency is decreased, display brightness in the pixels begins to creep, and these creeps are perceived as flickering. Another problem is that decreasing the refreshing frequency also decreases average electric potential in each frame period, which may lead to quality issues such as poor contrast.
Among many proposals currently made to achieve decrease in power consumption while avoiding these problems is a liquid crystal display device which includes a display unit provided with a memory section. The memory section digitally stores data which represents the image to be displayed. For example, Patent Literature 1 discloses a liquid crystal display device which includes an array substrate provided with a plurality of pixel groups arranged in a matrix pattern, and each pixel group is provided with a static memory. Such a liquid crystal display device as the above is capable of maintaining a constant pixel electrode potential without refreshing procedure, and therefore is capable of providing permanent display at low power consumption.